Plate and screw fixation of bones is a generally accepted method of treatment for several types of conditions that affect structural integrity of skeletal elements. A majority of plates are made from a malleable material such as a metal or metal alloy. One advantage of these types of materials is that they allow modification of the shape of the plate for more intimate contact with a bone surface which often may have a complex surface topography. Because of this need, it may often be required to bend bone plates along any of the three principle axes in order to provide a plate contour that more closely matches the surface anatomy at the site of application.
The majority of instruments used to bend or modify the shape of bone plates use some form of leverage to allow amplification of an applied manual force to cause bending of the typically much stronger metal plate. A basic bending tool is often a pair of flat bars of a given length. The flat bars at their ends capture a portion of a bone plate through an aperture or slot (see, for example, FIG. 1.). This type of tool, sometimes called a bending iron, is simple, requires relatively little space, and often is easily adapted to fit inside the utilized instrument or implant tray. This type of tool also has the advantage that it is light and simple for a user to manipulate. Bending irons are most effective for creating a vertical bend in a plate or producing a bend around an axis transverse to the long axis of the plate (see, for example, FIG. 1). This bending axis is resisted only by the thickness of the plate which is the plate's smallest dimension and therefore is the bending direction that requires the least amount of bending force.
The opening in the bending iron is a larger size than the dimensions of the plate itself in order to simplify insertion of the plate into the opening of the bending tool. As a result, this introduces some slop and rotational play between the opening and the implant. Fortunately, when a bending iron is used to bend a plate vertically around an axis transverse to the long axis of the plate, the plate seeks its most stable position in the opening of the instrument as the bending force is applied. This position of stability naturally aligns the bending instrument into a single, congruent plane and allows the applied force to be efficiently and completely transferred along this desired axis of bending. For this reason, these types of instruments work fairly well for creating plate bends in a vertical direction.
A bending iron is also generally effective for creating torsional bends around the long axis of a plate. Again, when used in this manner, as the torsional bending force is applied to the bending iron, the plate will twist within the opening of the slot or aperture until contact occurs along opposite diagonal edges of the plate in the opening. This natural position of stability again maintains the two bending tools along parallel but offset planes, allowing the application of force to be effectively and completely transferred to the plate around the desired torsional axis of bending. Although adding a torsional bend requires more force than applying a simple vertical bend as described previously, this feature can be overcome by providing a bending iron of sufficient length to gain the requisite mechanical advantage.
Unfortunately, plates sometime require the addition of a horizontal bend, around an axis that is perpendicular to the plane of the plate. In this direction of bending, bending irons typically do not work well, if at all. Because the slot or opening in the bending iron is oversized relative to the plate dimensions to allow easy insertion of the plate into the bending iron as well as accommodate other bends or contours of the plate, as bending force is applied to create a horizontal bend, the bending irons rotate around the axis of the plate to find the position of maximal stability. This causes the bending tools to stabilize in a position that is not opposite and coplanar, resulting in misdirection of the applied bending force in a direction that is outside the plane of the plate. This discrepancy in the direction of applied bending force and the desired direction of plate bending either creates a bending in a direction that is undesired or more commonly can prevent a bend from being created at all because bending is effectively applied over a larger dimension.
In addition, the torsional play of the plate within the opening of the bending tool results in a tendency for the end of the plate to slip out of the bending tool. Since this direction of bending is typically the most difficult, being through the width of the plate rather than the thickness, any torsional movement of the plate (1) reduces the effectiveness of maintaining all of the applied load along the desired axis of the bend, and (2) directs a bending axis that is oblique to any principle axis of the plate resulting in significant increase in the amount of applied load required to create a plate bend. Often, this may exceed the ability of a user to generate the required force.
Other bending instruments have been used, but require much heavier, bulkier, and more costly types of instruments. See, for example, FIGS. 2, 3, and 4 which show existing bending instruments that are currently in use (in addition to the instrument of FIG. 1). These instruments still have the problematic issue of controlling a bending force to be efficiently and effectively transferred to the plate in a horizontal direction (around the axis that is perpendicular to the plane of the plate). In addition, these instruments do not address the tendency for the plate to twist or slip out of the holding bracket while this type of force is applied.